The power factor (PF) of an AC-DC LED driver should be limited within a certain range to meet harmonic standard. A conventional AC-DC LED driver may be of a single-stage type or of a two-stage type. The single-stage type of AC-DC LED driver may have a large PF value by providing suitable parameters. However, control variables are selected in a limited scope, and may not be suitable for optimizing efficiency of the LED driver. An electrolytic capacitor should be included as an essential part, which, however, reduces lifetime of the LED driver and causes flicker which can be observed by human eyes. The two-stage type of AC-DC LED driver may be controlled with control variables in a full range, has a high PF value, and is suitable for optimizing efficiency of the LED driver, without flicker which can be observed by human eyes. The LED driver has an input for receiving a pulsed power supply and an output for providing a flat power supply, and has a storage capacitor in an intermediate bus for balancing the power supplies. If the storage capacitor is an electrolytic capacitor, it will adversely influence lifetime of the LED driver. If the storage capacitor is a ceramic capacitor or a thin-film capacitor, its operation voltage range will limit an intermediate bus voltage, or its footprint will be large for providing a large voltage range. The capacitor will increase cost of the LED driver if having a large footprint.
An AC-DC LED driver using multiple bus voltages has been proposed, which uses no electrolytic capacitor by means of power discretion control, and hence increases lifetime of the LED driver and has no problem of flicker. A control scheme for multiple-stage sequential LED strings configuration has been proposed by Seoul Semiconductor Co. Ltd., in which the LED strings are basically configured as shown in FIG. 1. The control scheme has a beneficial effect of avoiding flicker, but has an adverse effect of non-uniform usage of LEDs. For example, LED La maintains an on state when switch Sa is closed; LEDs La and Lb maintain an on state when switch Sb is closed; LEDs La, Lb and Lc maintain an on state when switches Sa and Sb are both opened. LED La has usage higher than that of LEDs Lb and Lc, which adversely influences lifetime of the whole system. Thus, the usage of LEDs is low or non-uniform in an inappropriate design of LED configuration scheme, which results in reduced lifetime of the whole system.